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United States Patent |
5,302,239
|
Roe
,   et al.
|
April 12, 1994
|
Method of making atomically sharp tips useful in scanning probe
microscopes
Abstract
An in situ plasma dry etching process for the formation of atomically sharp
tips for use in high resolution microscopes in which i) a mask layer is
deposited on a substrate, ii) a photoresist layer is patterned superjacent
the mask layer at the sites where the tips are to be formed, iii) the mask
is selectively removed by plasma etching, iv) after which the substrate is
etched in the same plasma reacting chamber, thereby creating sharp
microscope tips.
Inventors:
|
Roe; Fred L. (Boise, ID);
Tjaden; Kevin (Boise, ID)
|
Assignee:
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Micron Technology, Inc. (Boise, ID)
|
Appl. No.:
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884482 |
Filed:
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May 15, 1992 |
Current U.S. Class: |
216/11; 216/57; 216/67; 216/79; 216/99; 250/306; 250/311 |
Intern'l Class: |
G21K 007/00 |
Field of Search: |
156/657,659.1,661.1,662,643,646
250/306,311
|
References Cited
U.S. Patent Documents
4310380 | Jan., 1982 | Flamm et al. | 156/657.
|
4639288 | Jan., 1987 | Price et al. | 156/657.
|
4741799 | May., 1988 | Chen et al. | 156/657.
|
4968382 | Nov., 1990 | Jacobson et al. | 156/657.
|
4968585 | Nov., 1990 | Albrecht et al. | 156/643.
|
4986877 | Jan., 1991 | Tachi et al. | 156/657.
|
5051379 | Sep., 1991 | Bayer et al. | 156/643.
|
5066358 | Nov., 1991 | Quate et al. | 156/657.
|
5082524 | Jan., 1992 | Cathey | 156/643.
|
5094712 | Mar., 1992 | Becker et al. | 156/643.
|
5201992 | Apr., 1993 | Marcus et al. | 156/643.
|
Other References
Marcus et al., "Formation of Silicon Tips with 1 nm Radius", Appl. Physics
Letter, vol. 56, No. 3, Jan. 15, 1990.
Hunt et al., "Structure and Electrical Characteristics of Silicon
Field-Emission Microelectronic Devices", IEEE Transaction on Electron
Devices, vol. 38, No. 10, Oct. 1991.
McGruer et al., "Oxidation-Sharpened Gated Field Emitter Array Process",
IEEE Transactions on Electron Devices, vol. 38, No. 10, Oct. 1991.
Farooqui et al., "Microfabrication of Submicron Nozzles in Silicon
Nitride", Journal of Microelectromechanical System, vol. 1, No. 2, Jun.
1992, pp. 86-88.
|
Primary Examiner: Dang; Thi
Attorney, Agent or Firm: Pappas; Lia M.
Claims
We claim:
1. A process for the formation of microtips, said process comprising the
following steps:
exposing a patterned substrate to a plasma, thereby creating microtips
having a tip size less than 10.ANG.; and
segmenting the substrate in order to separate at least some of the
plurality of microtips from each other.
2. The process according to claim 1, wherein said plasma comprises a
halogenated species.
3. The process according to claim 2, wherein said halogenated species
comprises at least one of chlorine and fluorine.
4. The process according to claim 1, wherein said patterned substrate
comprises a mask layer and a photoresist layer.
5. The process according to claim 4, wherein said mask layer is an oxide.
6. The process according to claim 5, further comprising the steps of:
stripping said mask layer after subjecting said substrate to said plasma.
7. The process according to claim 6, wherein said stripping step is a wet
etch, said wet etch comprising hydrogen fluoride.
8. A method for controlling aspect ratios useful for forming very sharp
asperities comprising the following step:
isotropically etching a substrate having a mask layer disposed thereon,
thereby forming said sharp asperities in a single etch step, said sharp
asperities having a tip size of less than 10.ANG..
9. The process according to claim 8, wherein a plasma is used to etch said
substrate, said plasma comprising etchant gases, said aspect ratio being
controlled by said etchant gas flow.
10. The process according to claim 9, wherein said mask layer is an oxide.
11. The process according to claim 10, wherein said etchant gases comprises
halogenated species.
12. The process according to claim 11, wherein said halogenated species
comprising at least one of chloride and fluorine.
13. The process according to claim 12, further comprising the step of:
removing said mask layer using a wet etch, said wet etch comprising
hydrogen fluoride.
14. The process according to claim 13, further comprising the step of:
cooling said substrate while said substrate is being etched.
15. The process according to claim 14, wherein a photoresist layer is used
to pattern said mask layer.
Description
FIELD OF THE INVENTION
This invention relates to high precision mechanical movements, such as used
in high resolution microscopes, and more particularly, to a process for
the formation of very sharp tips or edges used in scanning probe
microscopes.
BACKGROUND OF THE INVENTION
The present invention uses a substrate which, in the preferred embodiment
includes a silicon layer. However, a deposited material, such as
polysilicon or amorphous silicon, may also be used. Typically, these are
semiconductor wafers, although it is possible to use other materials, such
as silicon on sapphire (SOS) . Therefore, "wafers" is intended to refer to
the substrate on which the inventive tips are formed.
High resolution microscopes play an important role in research by providing
an image of objects previously imperceptible. The "scanning probe"
microscopes are becoming more important in industry as such microscopes
have several advantages over the scanning electron microscopes (SEM)
presently used. Some of the advantages include: superior resolution,
minimal sample damage, and the scanning probe microscopes provide
quantitative, three-dimensional topographic data. The scanning tunneling
microscope (STM), and atomic force microscope (AFM) are among the
currently available high resolution probe scanning microscopes.
These high resolution microscopes function by means of a cantilever system.
A very fine, sharp tip is disposed on one end of a soft cantilever spring
on the other end of the cantilever is a mechanism for sensing the
cantilever's deflection. A feedback loop monitors and controls the
deflection. The microtip is used to scan a surface. A mechanical scanning
system moves the surface with respect to the tip in a raster pattern. When
the very fine tip encounters a microscopic bump in the surface, the
sensing mechanism records the distortion, thereby producing an image of
the surface as the surface is scanned on a display system that converts
the measured data into an image.
Very fine, extremely sensitive tips are required for clarity in the high
resolution microscopes. Current tips can have a radius less than 400.ANG.,
and are square-pyramidal in shape. The multiple contact points on the tip
can result in atomic size images which show a complex superposition of
effects.
Some of the present microscopes employ tips made of tungsten, which tips
are further coated with a layer of silicon. One drawback with such
tungsten tips is the difficulty in depositing the silicon in the desired
thinness.
Other tips for scanning tunneling microscopes are disclosed in U.S. Pat.
No. 4,985,627 entitled, "Spin-Polarized Scanning Tunneling Microscope,"
and U.S. Pat. No. 4,968,585 entitled, "Microfabricated Cantilever Stylus
with Integrated Conical Tip; Semiconductor Integrated Circuit Fabrication
Techniques for Tip for Scanning Tunneling Microscope."
In contrast to the prior art, tips fabricated by the process of the present
invention are approximately 7.ANG.-10.ANG. at the apex. The process of the
present invention employs dry etching (also referred to as plasma etching)
to fabricate sharp tips. Plasma etching is the selective removal of
material through the use of etching gases. It is a chemical process which
uses plasma energy to drive the reaction. Those factors which control the
precision of the etch are the temperature of the etchant, the time of
immersion, and the composition of the gaseous etchant.
Various papers refer to reactive ion etching (RIE) and orientation
dependent etching (ODE) of silicon to form tips. These technologies rely
on either expensive multiple deposition and evaporation steps, or dry etch
processes bound by the isotropic etching characteristics of the process
gases. For example, prior art dry etch processes limit the manufacturer to
a height to width etch ratio of 1:1. To alter this 1:1 ratio in order to
obtain an increased depth, a deeper mask would be required.
SUMMARY OF THE INVENTION
The process of the present invention involves an in situ plasma etch of a
silicon substrate upon which has been deposited a hard mask layer and a
patterned photoresist layer. The mask layer is etched to expose the
silicon substrate, which silicon substrate is then etched to form the
sharp tips. Alternatively, the patterned layer can have the dual function
of hard mask layer and photoresist layer.
The process of the present invention can be used to produce atomically
sharp tips with relatively any given aspect ratio and height with a single
step (in situ) plasma dry etch process. The elimination of steps in a
manufacturing process represents a tremendous advantage both in time and
money. Further, the less handling of the wafers that is required, the
greater the yields which tend to result.
Although the preferred embodiment is a single step process, the process of
the present invention can also be carried out in a series of steps whereby
the ratio of reactant gases, the power supplied, or the pressure applied,
is varied.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be better understood from reading the following
description of nonlimitative embodiments, with reference to the attached
drawings, wherein:
FIG. 1 is a cross-sectional schematic drawing of a high resolution scanning
probe microscope tip fabricated by the process of the present invention;
FIG. 2 is a cross-sectional schematic drawing of a substrate on which is
deposited a hard mask layer and a patterned photoresist layer;
FIG. 3 is a cross-sectional schematic drawing of the structure of FIG. 2,
after the mask layer has been selectively removed by plasma dry etch;
FIG. 4 is a cross-sectional schematic drawing of the structure of FIG. 3,
after undergoing a silicon etch; and
FIG. 5 is a cross-sectional schematic drawing of the structure of FIG. 4,
depicting the sharp tip after the silicon etch has been completed, and the
mask layer has been removed.
DETAILED DESCRIPTION OF THE INVENTION
For the purpose of providing background material and illustrating the state
of the scanning probe microscope art, the following articles are hereby
incorporated by reference herein:
Rugar, Daniel and Hansma, Paul, "Atomic Force microscopy," PHYSICS TODAY,
October 1990. pp. 23-30.
Howland, R.S., "Scanned Probe Microscopy for Semiconductor Inspection,"
TIME & MEASUREMENT WORLD, January 1991, pp. 53-57.
Smith, Ian and Howland, Rebecca, "Applications of Scanning Probe Microscopy
in the Semiconductor Industry, SOLID STATE TECHNOLOGY, December 1990, pp.
53-56.
Referring to FIG. 1, probe tip 13 is disposed on cantilever 15, which
cantilever is connected to a spring deflection sensor 17. As the sample 20
is mechanically moved in a raster pattern under microtip 13 short range
interatomic forces repel the tip 13, and the cantilever 15 is deflected,
which deflection is montiored by a feedback system in sensor 17, and then
converted through a display system (not shown) which display system
provides an image of the sample 20. The sensor 17 can be a position
sensitive, photodiode detector. Cantilevers 15 are approximately 100 .mu.m
to 200 .mu.m long, and 0.5 .mu.m thick, and made from silicon nitride.
The process of the present invention yields atomically sharp tips 13. For
purposes of this application, "atomically sharp" refers to a degree of
sharpness that can not be defined clearly by the human eye when looking at
a scanning electron microscope (SEM) micrograph of the structure. In other
words, in a SEM micrograph of the microtip 13, the human eye can not
adequately distinguish where the peak of the tip 13 actually ends because
the peak of the tip 13 is of finer dimensions than the clarity or
resolution capable with the SEM, and therefore the tip 13 appears somewhat
blurred. In reality, the apex of the tip 13 is approximately
7.ANG.-10.ANG. across.
Experimental results have yielded tips 13 having base widths of
approximately 1 .mu.m and heights in the range of 2 .mu.m. Further
experimentation is anticipated to yield tips 13 having base widths in the
relative range of 0.75 .mu.m to 1.25 .mu.m, and relative heights in the
approximate range of 0.75 .mu.m to 2.5 .mu.m or more. In the process of
the present invention, the balancing of the gases in the plasma etch will
enable the manufacturer to determine, and thereby significantly control,
the dimensions of the tip 13. Therefore, tips 13 which are taller than 2.5
.mu.m are conceivable using the process of the present invention and the
correct etchant gas ratio (e.g. Cl.sub.2 :NF.sub.3 ratio) . The greater
the ratio of the gases, the taller the resulting tip 13. It is anticipated
that in actual production, the tips will have a height of approximately
1.5 .mu.m to 7.5 .mu.m.
FIG. 2 depicts the substrate 11, which substrate can be amorphous
polysilicon, polysilicon, or any other material from which the tip 13 can
be fabricated. The substrate 11 has a mask layer 30 deposited or grown
thereon. The hard mask layer 30 can be made of any suitable material which
is selective to the substrate 11, the preferred material being an oxide,
typically silicon dioxide.
A photoresist layer 32 is patterned on the mask layer 30. Photoresist 32 is
commonly used as a mask during plasma etch operations. For etches of
silicon, silicon dioxide, silicon nitride, and other metallic and
non-metallic compounds, photoresist 32 displays sufficient durability and
stability.
Alternatively, a hard mask using only a single photoresist layer 32 can be
used. In such a case, an oxide layer would not be needed. The use of a
photoresist layer 32 alone is not the preferred method as greater
selectivity during the silicon substrate 11 etch is currently available
using an oxide layer 30.
The next step in the process is the selective removal of the oxide mask 30
which is not covered by the photoresist pattern 32 (FIG. 3). The selective
removal of the hard mask 30 is accomplished preferably through a dry
plasma etch, but any oxide etch technique can be used.
In a plasma etch method, the typical etchants used to etch silicon dioxide
include, but are not limited to: chlorine and fluorine, and typical gas
compounds include: CF.sub.4, CHF.sub.3, C.sub.2 F.sub.6, and C.sub.3
F.sub.8. Fluorine with oxygen can also be used to accomplish the oxide
mask 30 etch step. In our experiments CF.sub.3, CHF.sub.3, and argon were
used. The etchant gases are selective with respect to silicon, and the
etch rate of oxide is know in the art, so the endpoint of the etch step
can be calculated.
In the preferred embodiment, the photoresist layer 32 does not have to be
stripped because the photoresist layer 32 is removed in situ during the
plasma etch of the substrate 11. Note however, that in changing the
balance or ratio of the process etch gases, that the removal rate of the
photoresist 32 also changes, and therefore, a removal step of any
remaining photoresist 32 may be necessary post-etch. Removal of the
photoresist layer 32 can be accomplished by any of the methods known in
the art.
Immediately after the oxide etch step, preferably in the same chamber and
using the same cathode, the silicon layer 11 is etched, this generates a
profile as depicted in FIG. 4. Fluorine (preferably NF.sub.3, but any
fluorine containing process gas can be used) and chlorine (preferably
Cl.sub.2, but any chlorine containing process gas can be used) are
combined in a plasma etching system to create the sharp tips 13 used in
field emitting devices. Other silicon etchants include: CF.sub.4,
SiF.sub.4, CHF.sub.3, and SF.sub.6, and other typical gas compounds
include: BCl.sub.3, CCl.sub.4, SiCl.sub.4, and HCl.
An alternative embodiment involves removing the substrate 11 from the
plasma reactor after the mask layer 30 has been etched, and then placing
the substrate 11 in a second plasma reactor to accomplish the silicon
substrate 11 etch. In other words, the process of the present invention
need not be carried out in situ, although the in situ method would be the
most efficient.
The following are the ranges of parameters for the process described in the
present application. Included is a range of values which we investigated
during the characterization of the process as well as a range of values
which provided the best results for tips 13 that were from 1.5 .mu.m to 2
.mu.m high and 0.75 .mu.m to l .mu.m at the base. One having ordinary
skill in the art will realize that the values can be varied to obtain tips
13 having other height and width dimensions.
______________________________________
INVESTIGATED PREFERRED
PARAMETER RANGE RANGE
______________________________________
Cl.sub.2 20-70 SCCM 40-60 SCCM
NF.sub.3 3-15 SCCM 8-12 SCCM
Cl.sub.2 :NF.sub.3
23:1-1.3:1 7.5:1-3.3:1
POWER 100-500 W 200-300 W
PRESSURE 50-300 mTORR 160-200 mTORR
TEMPERATURE 20.degree. C. 20.degree. C.
______________________________________
In the preferred embodiment of the process, the substrate is kept at a
temperature of approximately 20.degree. C. through "backside cooling,"
which is done by cooling the chuck upon which the wafer rests.
Although we only used a 20.degree. C. wafer temperature, the process of the
present invention can be used over a wide range of temperatures. A higher
temperatures, the Cl.sub.2 :NF.sub.3 ratio would have to be increased in
order to maintain the tip 13 height, and at still higher temperatures one
may have to use a combination of F and Br, Cl and Br, or F and Cl and Br
in order to maintain the tip 13 height due to the increase in volatility
of the etch products (e.g. SiF.sub.4 and SiCl.sub.4) at higher
temperatures.
In other words, the temperature dependence of the volatilities of the etch
products (for example, SiF.sub.4 and SiCl.sub.4) is important. Changing
the temperatures, can change the volatilities, and therefore the height
and width ratio.
While the invention is presently in the developmental stage, it is
anticipated that the inventive process will include a low pressure
atmosphere in order to produce a faster oxide etch rate. Low pressure
allows for more ion bombardment because of the longer mean free path that
the ions have before colliding with the surface, or other ions. When
combined with high radio frequency (RF) power, the etch rate is increased.
Low pressure and RF power do have drawbacks, however. Although RF induced
ion bombardment assists in oxide etch, it also contributes to photoresist
erosion, which is undesirable. Further, if RF power is too high, the
resist will "burn" or reticulate.
The use of a low pressure process for etching oxide in the present
invention overcomes the negative effects mentioned above by the use of a
magnetic field and helium cooled wafers.
Any combination of halide (e.g. fluorine, chlorine, bromine, etc.)
containing etch process gases can be used for which the etch products
resulting from the plasma assisted reaction of the reactant process gases
and the substrate have significantly different volatilities (also referred
to as vapor pressure) at the temperature at which the etch takes place.
The ratio of the halide containing process gases is used to control the
degree of isotropy or anisotropy (perfect anisotropy creating
substantially vertical sidewalls) , and the height and width at the base
of the cathode tip 13.
The degree of isotropy (also referred to as the degree of undercut) is a
product of the differing volatilities of the different etch products. For
example, in our etch using fluorine (in the form of NF.sub.3) and chlorine
(in the form of Cl.sub.2) the resulting etch products, SiF.sub.4 and
SiCl.sub.4, have different volatilities, and therefore evaporate at
different rates, thereby determining the height to width ratio. Different
ratios of fluorine to chlorine yield different ratios of height to width.
The primary means of controlling the height to width ratio of the tip 13
formed by the process of the present invention is through the combination
of halide containing gases. However, by making use of the temperature
dependence of the evaporation rate of the etch products in combination
with the increased removal rate of the etch products in a directional way
(due to the directional nature of plasma created ions "sputtering" off the
etch product) . One may control the height to width ratio of the tip 13 by
controlling the temperature and/or the impact energy of the ions in the
plasma. Ion impact energy is increased by raising the RF power or lowering
the process pressure (this increases the mean free path as described
above).
The process of the present invention is dependent upon the combination of
two different gases having good selectivity with respect to the oxide mask
30. In such a case, the etch will not be bound by the normal height to
width etch ratio of 1:1, but the etch can be controlled through the gas
flow, i.e. the ratio of fluorine to chlorine. The degree of the undercut
(also referred to as isotropy) can be substantially controlled by
regulating the amount and partial pressure of the reactant etching gases.
The amount of power to be supplied, and hence, the RF field or magnetic
field created by the power supply depends on the flow of the etchant gases
selected, which flow is dependent on the size and sharpness of the tips 13
desired.
One having ordinary skill in the art will realize that the other
frequencies of energy (e.g. microwaves) other than RF could be adapted for
use in the process of the present invention. Further, although the plasma
etches of the present invention were carried out in a reactive ion etch
(R.I.E.) reactor, a cyclotron could be used, as well.
After the tip 13 is fabricated, and the desired dimensions have been
achieved, the oxide mask layer 30 can be removed, as depicted in FIG. 5.
The mask layer 30 can be stripped by any of the methods well known in the
art, for example, a wet etch using a hydrogen fluoride (HF) solution or
other HF containing mixture. In the preferred embodiment, the mask layer
30 and the photoresist layer 32 will be substantially consumed by the
process of the etch, and the substrate 11 can be dipped in a HF bath.
During the silicon substrate 11 etch, the mask layer 30 and photoresist
layer 32 may simply fall off the tip 13 as the tip 13 becomes sharper.
After the tips 13 have been fabricated, the wafer substrate 11 can be cut
with, for example a diamond-tipped microsaw, and the individual tips 13
separated. Each tip 13 can then be adhered to a cantilever 15 by any of
the methods known in the art, as by way of examples, adhesive or a frit
seal.
All of the U.S. patents and patent applications cited herein are hereby
incorporated by reference herein as if set forth in their entirety.
While the particular process for creating sharp tips for use in flat panel
displays as herein shown and disclosed in detail is fully capable of
obtaining the objects and advantages herein before stated, it is to be
understood that it is merely illustrative of the presently preferred
embodiments of the invention and that no limitations are intended to the
details of construction or design herein shown other than as described in
the appended claims. For example, the process of the present invention was
discussed with regard to the fabrication of sharp scanning probe tips for
use in high resolution microscopes, however, one with ordinary skill in
the art will realize that such a process can be applied to other imaging
systems using highly sensitive probes.
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